A system and method are provided for implementing an asynchronous buffering module and/or scheme employing an integrated registration function, particularly for in-line printing in an image forming system. Multiple simplex marking engines are arranged in series and provided with a mechanism that includes an asynchronous buffering module with the capacity to modify the timing of individual sheets of image receiving media passing between the in-line marking engines. The buffering module is augmented with an integrated registration function for the in-line duplex printing. The combined mechanism provides a capacity between the multiple marking engines to effect timing synchronization and re-registration for the sheets exiting the first serial marking engine allowing them to be properly timed and registered for feeding to the second serial marking engine. An asynchronous nature of the combination of media marking engines is synchronized in the feed of the sheets between the media marking engines.